There are times (and quests) when "42" is not a sufficient answer. This quest is such (a quest, not a time, of course!). How to solve this quest, then?
While heroes tend to do so by killing monsters and more monsters, we shall think about each of the questions for itself:

The heroes: The heroes can be excluded as they neither possess the necessary intelligence to host such a what. Also, they wouldn't have to solve the quest, then.

The traders: Traders can be excluded as well, as the quest has got a reward. Were traders involved, it wouldn't.

The monsters: Although it is known that there can be friendships between monsters and heroes, their relationship is tensed. Apart from random encounters on the street, there could never be such a what featuring monsters and heroes.

The boss-monsters: Boss-monsters aren't evil. They just don't like being disrupted in their personal area. Usually, they are great hosts! Is it really that improbable that a nice skilled boss-monster celebrated a what and thanked a hero for coming with a self-created Wonderbox?

What?What does a hero aim for in life? Correct, beer! The boss-monster hosts a party with loads of beer!

It's a long road to find the answers.

When?In the second section of this quest chain, Go there, do that, get the t-shirt, the hero seems to finally go there. If we now add the estimated durations of these quests, assuming ten minutes per quest, we get:

10min + 10min = 20min

This means the party will take place twenty minutes after the beginning of quest of the quest Find out who, what, when, where, why, and how. With

t = x + 20min; mod 1440

where x equals the time of day and

[x] = min

one can easily calculate the exact time of the party.

Why?As mentioned in the introduction, "42" is not always a sufficient answer. Here it is, though.

How?This interrogative particle doesn't start with a "w" and is thus, according to §54b, 74-85, not valid.